FTIR for Light and Elevated Temperature Induced Degradation (LeTID) in Solar Cells

Fourier Transform Infrared spectroscopy offers inline solutions for chemical bonding, epi thickness, and trench depth measurements. Through optical modeling of the transmission or reflectance spectra, information about the electronic structure and chemical composition may be obtained, which can be used for process control and monitoring. In this article, we demonstrate the measurement capabilities of FTIR for the hydrogen bonding in cell silicon nitride and amorphous carbon hard masks (ACHM), which are used for 3D NAND fabrication. For cell silicon nitride, deconvolution of the spectra allows differentiation between individual peaks corresponding to Si-N, Si-H, N-H, Si-O, and Si-OH bonds. This differentiation identifies wafers with varying hydrogen content and distinct processes. Similarly, for ACHM, peak areas related to sp² C-H bonds and aromatic C=C bending reveals the hydrogen skew conditions in three wafers. Notably, a linear relationship between high broadband absorption and low C-H bonds (and aromatic C=C) peak area is observed. The measurements exhibit good repeata... Read More

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Accurately measuring the hydrogen content in silicon (Si) solar cells is essential due to its connection to surface degradation and light and elevated temperature induced degradation (LeTID). Fourier Transform-Infrared (FT-IR) spectroscopy provides a quantitative technique for determining the content of various hydrogen species in Si wafers that have undergone various process steps. In this study, we examine both the effect of a silicon nitride (SiNx:H) layer during FT-IR spectroscopic measurements on hydrogen species, as well as the impact of an emitter present during firing on the amount of hydrogen introduced into Si wafers. We find that the presence of SiNx:H during measurements has negligible effects on the measured hydrogen species, potentially simplifying the preparation steps for FT-IR. For the emitter investigation we analyze boron (B)- and gallium (Ga)-doped p-type wafers to detect H-B, H-Ga, Oi-H2, and H2. We observe that hydrogen species initially present in B- and Ga-doped Si wafers differ significantly. Only H-Ga is detected in Ga-doped wafers, while H-B, Oi-H2, and H2 ... Read More

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The hydrogenation step contributing to the high efficiencies (>25%) reached with poly-Si/SiOx passivated contacts solar cells is still poorly understood. In this study, Fourier transform infrared spectroscopy (FTIR) is used to follow the different bonding configurations of H during the fabrication process. The carrier lifetime degradation upon annealing is correlated to an important loss of SiH bonds, from both the aSi:H film and the SiOx interfaces. The subsequent hydrogenation step results in the formation of a small number of SiH bonds near the crystalline silicon c-Si/SiOx interface, associated with the low stretching mode (LSM) and correlated to a significant lifetime improvement. These bonds feature a preferential orientation, as shown by polarized measurements.

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In recent years, the formation of microstructures on silicon wafer has gained popularity as a concept for increasing photon trapping and light absorption for optoelectronics applications. This study used three methods to improve infrared light absorption in silicon samples - sample preparation, Radio Corporation of America (RCA) cleaning, and chemical wet etching. The solutions used for Radio Corporation of America (RCA) clean were water (H2O), Ammonium hydroxide (NH4OH), hydrogen perioxide (H2O2), Hydrofluoric acid (H.F.). Three silicon wafers with a 1cm2 orientation were cut and cleaned using RCA, and then surface-textured using a wet chemical procedure by etching into different chemical solutions of Sulfuric acid (H2SO4) of the same concentration. The wafers were removed at different etching time intervals (5, 10, 15 minutes) and analysed using an infrared spectrometer with Fourier transformation (FTIR) to study the absorptions of light. A mean absorbance of 0.9801 a.u, 0.9845 a.u and 0.977 a.u for 5, 10 and 15 minutes of texturization was obtained. The results showed a wafer that... Read More

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The influence of the thickness of the hydrogen-rich silicon nitride layer on the amount of hydrogen introduced into the bulk of silicon wafers, with boron or gallium as the acceptor dopant species, is investigated using cryogenic Fourier Transform-infrared spectroscopy.Nitride layers with comparable refractive indices are deposited on Czochralski wafers and subjected to a simulated contact firing process.Thus, hydrogenation of wafers is performed with different thicknesses of the respective hydrogen sources.Fourier Transform-infrared spectroscopy at 5.0 K show that the hydrogen concentrations can be varied by altering the film thickness.The effect of a subsequent passivation process, i.e. deposition of a second hydrogen rich dielectric film at an elevated temperature, is also investigated.We observe that the passivation may alter the states of hydrogen in the bulk silicon or cause unintentional introduction of hydrogen.This passivation process also revealed considerable differences between boron and gallium doped wafers; The hydrogen-boron concentration grew more than the correspondi... Read More

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Photons of varying wavelengths exert substantial effects on silicon heterojunction (SHJ) solar cells. Collaborative research previously establishes that light soaking with longwavelength photons can activate boron doping in hydrogenated amorphous silicon (aSi:H), thereby augmenting cell efficiency (Eff). Herein, this investigation is extended, exploring the effects of shortwavelength photons on aSi:H layers, SHJ solar cells, and modules. The ultraviolet A (UVA) light with the wavelengths peak of 365 nm can disrupt SiH bonds, resulting in a notable reduction in hydrogen content within both intrinsic and doped aSi:H films, and the deterioration of deteriorated interface passivation. Following exposure to 60 kWh m 2 of UVA light, both Eff and module power output decrease significantly, primarily attributable to the degradation of opencircuit voltage and fill factor. As a feasible solution, the application of a lightsoaking process or the implementation of UV band cutoff module encapsulants can effectively mitigate the loss induced by UV irradiation, thereby ensuring the longte... Read More

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In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light-and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, lowtemperature Fourier-transform infrared spectroscopy, and fourpoint-probe resistance measurements.Degradation conditions were light soaking at 77 C and 1 sun-equivalent illumination intensity and dark anneal at 175 C.The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B-or P-doped).We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp.This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H 2 , and to a smaller degree the concentration of boron-hydrogen (BH) pairs.A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si.In p-type Si, the LeTI... Read More

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In current silicon solar cell technologies, hydrogen is incorporated into the solar cell during the fast-firing process. It passivates defects at the surface and in the bulk, but also leads to light- and elevated-temperature-induced degradation (LeTID). Although it is known that the hydrogen content and the LeTID extent can be reduced by employing a slower cooling ramp during the fast-firing process, the exact mechanism behind this phenomenon remains unclear. This study aims at closing this gap by investigating the impact of cooling ramps with different temperature plateaus on hydrogen (complexes) in B-doped FZ-Si wafers. The fired wafers are analyzed with FT-IR spectroscopy, four-point-probe resistivity measurements, and LeTID tests via effective lifetime measurements. Our findings provide evidence that hydrogen not only diffuses into the silicon bulk but can also effuse out of it during the cooling ramp. A one-dimensional hydrogen model is built in Sentaurus TCAD to simulate the in- and out-diffusion of hydrogen and to compare it with the experimental results. The experimentally de... Read More

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The influence of the cooling rate during the fast-firing process and of the sample thickness on the initial hydrogen (complex) distribution in p- and n-type silicon wafers is investigated using low-temperature Fourier Transform-Infrared (FT -IR) spectroscopy. The impact of the introduced hydrogen on the formation of defects during dark annealing and light soaking is then studied by resistivity and charge carrier lifetime measurements. We observe a lower overall hydrogen concentration for thinner wafers or slower cooling rates. This is especially pronounced for the concentration of the hydrogen molecule H2A. We observe a weak signature of light- and elevated-temperature-induced degradation (LeTID) during dark annealing accompanied by a significant increase in BH pair concentration. Interestingly, the extent of degradation does not correlate with the chosen process variations. Regeneration of the carrier lifetime occurs earlier in thinner wafers and in fast-fired samples. During light soaking, the LeTID extent clearly correlates with the initial hydrogen (H2A) content, while the BH pai... Read More

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Hydrogen-boron (HB) pairs and hydrogen dimers (H2) can be measured in silicon wafers directly using cryogenic Fourier Transform-Infrared (FT-IR) spectroscopy, or indirectly through a resistivity change upon annealing. The change in the HB and the H2 concentrations during annealing of hydrogenated float zone silicon wafers shows good agreement with both approaches, both with respect to absolute values as well as the temporal evolution. Thus, the model where H2 dissociates and provides a source of H+ that can passivate B and form HB pairs is supported. FT-IR measurements show that there is a small concentration of HB pairs in the wafers prior to annealing, presumably depending on the firing process used. In addition, the total hydrogen concentration visible using FT-IR is not constant, indicating the presence of a hydrogen state that is hidden from detection optically, while still contributing to the passivation of acceptors.

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We demonstrated evaluation of sub-gap state and gap-state defect in hydrogenated amorphous silicon oxide (a-SiOX:H) photo-absorber within solar cell structure from internal quantum efficiency (IQE) measured by Fourier transform photocurrent spectroscopy (FTPS). In IQE spectra for a-SiOX:H thin-film solar cells, exponential tail and IQE corresponding to gap-state defect was observed. We also investigated light-induced degradation in a-SiOX:H photo-absorber within solar cell structure by FTPS. IQE related to gap-state defect increased and conversion efficiency decreased by light irradiation, which corresponds to light-induced degradation. Urbach energy obtained from IQE spectra increased by light irradiation.

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The periodic density functional theory (periodic DFT) method was employed for the interpretation of infrared radiation spectra (IR spectra) measured for the hydrogen-covered H/Si(100) surface after the standard step of native oxide removal by brief etching in 40% NH4F. The IR employed the attenuated total reflectance (ATR) method. The periodic DFT calculations of IR spectra focused on reconstructions of H/Si(100) that involved combinations of surface-terminating SiH groups including the double-occupied dimer (DOD), dihydride (DH), and trihydride (TH). The IR spectra calculated with periodic DFT for H/Si(100) surfaces were compared with the IR spectra calculated by means of DFT in SiH clusters. The periodic DFT provided considerably better and more reliable theoretical description of the IR spectra by keeping the periodicity of the silicon material that guaranteed proper spatial distribution of the SiH species within H/Si(100). The calculated IR spectrum for H/Si(100) that involved a combination of two DOD and three DH groups (7 1 reconstruction) was in good agreement with the me... Read More

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Light injection and cooling conditions can induce some differences in the temperature-raising time of multi-crystalline silicon (mc-Si) solar cells. Through real-time tracking and monitoring of sample surface temperature, the temperature-raising steps for achieving the optimum efficiency improvement and degradation mitigation were optimized and modified. Then the corresponding hydrogenation treatments under a high-intensity infrared LEDs source were carried out based on improved steps. The results indicated that the optimal temperature-raising time should be manipulated at around 60 s and then followed by a 2-min hydrogenation treatment. Furthermore, the temperature-raising process should keep the temperature rising continuously without interruption or temperature drop. However, excessive thermal treatment time damaged the formation of defect precursors and extended the hydrogenation time. Moreover, the Fourier Transform Infrared spectrum curves illustrated that the peak intensity of the SiH bonds treated at the 60-s temperature-raising time was more significant than that of other t... Read More

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Method for evaluating carbon concentration in silicon samples using hydrogen introduction and thermal treatment, enabling assessment of carbon contamination in silicon wafers and single crystal ingots. The method involves introducing hydrogen into the silicon sample, heating it to a controlled temperature, and analyzing the resulting trap levels to determine carbon concentration. This approach enables process control and optimization in silicon wafer manufacturing and single crystal ingot growth.

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Hydrogen (H) is thought to be strongly involved in the light and elevated temperature-induced degradation observed predominantly in p-type silicon wafers, but the nature of the defect or defects involved in this process is currently unknown. We have used infrared (IR) spectroscopy to detect the vibrational signatures due to the HB, HGa, and H2*(C) defects in thin, hydrogenated, p-type multicrystalline silicon wafers after increasing the optical path length by preparation and polishing the edges of a stack of wafers. The concentrations of the HB and HGa acceptor complexes are reduced to 80% of their starting values after low intensity (5 mW/cm2) illumination at room temperature for 96 h. Subsequent high intensity illumination (70 mW/cm2) at 150 C for 78 h further decreases the concentrations of these defects; to 40% (HB) and 50% (HGa) of their starting values. Our results show that, with careful sample preparation, IR spectroscopy can be used in conjunction with other techniques, e.g., quasisteady-state photoconductance, to investigate the involvement of different H-related ... Read More

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